The molecules involved in cell movement and changes of cell shape assume a variety of supramolecular forms during the cell cycle. For example, actin exists as monomers, filaments, filament-bundles and apparently as part of mitotic spindles. These assemblies are transient and their formation must be under strict cellular controls. The filament-bundles appear to be part of a membrane-associated cytoskeleton which may be related, not only to cell movements and cell shape, but also to cell adhesion and contact inhibition. Transformation of cells by oncogenic viruses results in changes in these membrane properties as well as in the structure of the cytoskeleton. Our approach is to purify the components involved in these cellular events and to study them biochemically and structurally, with emphasis on factors controlling their interaction and their assembly into higher order supramolecular forms. We purified and partially characterized actin and myosin from amoebae of Dictyostelium discoideum and have shown that some of the actin copurifies with the cell membrane. Distyostelium actin forms filament-bundles in vitro when mixed with a minor component from the amoebae. Purification and characterization of this factor which controls bundle formation is underway. We recently isolated the intact cytoskeleton of normal and transformed chick embryo fibroblasts by gentle lysis of Triton X-100. We propose to examine both biochemical and structural changes in these cytoskeletons upon transformation of cells by Rous sarcoma virus. Furthermore, we intend to explore whether the three major protein components of these cytoskeletons interact with one another. In addition, we are working with developing eggs of the sea urchine Strongylocentrotus purpuratus. We propose to examine biochemically the actin which we find associated with spindles isolated from metaphase eggs. Furthermore, the synchronous development of a population of these eggs allows us to examine the changing pattern of cortex-associated actin during the cell cycle.